Signaling system



July 22, 1941. s. F. NELSON SIGNALING SYSTEM Filed June 27, 1940 FIG.

FIG-Z INVENTOR j S. ENEL SON A 7" TORNE V Patented July 22, 1941 SIGNALING SYSTEM Stanley F. Nelson, Eustis, Fla., assignor to Bell Telephone Laboratories; Incorporated, New York, N. Y., a corporation of New York Application June 27, 1940, Serial No. 342,633

6 Claims.

This invention relates to selective signaling systems and more particularly to selective ringing systems for two-party telephone lines.

It is common practice in two-party telephone line circuits to connect the ringer of one station L between the ring conductor of the line and ground, and the ringer of the other station between the tip conductor of the line and ground, and to provide switching means at the centralofiice for selectively connecting a grounded source of ringing current to either the ring or tip conductors and ground to the other conductor. In this manner the stations on a two-party line may be selectively signaled.

It is well known that a condition of crossring may be produced in such systems should the party at a called station remove the receiver from its switchhook during the time signaling current is being applied to the conductor to which the ringer at the called station is connected. When the called subscriber removes the receiver, the tip and ring conductors are connected together at the switchhook contacts through the station telephone set, and the signaling current being transmitted over one conductor finds a path to ground through the ringer at the other station. This path through the ringer at the unsignaled station is in parallel with the. normal return path to ground at the central office over the other conductor so that the ringer included therein is subjected to the potential drop over that portion of the said other conductor extending between ground at the central olfice and the point on the conductor at which the ringer at the unsignaled station is connected. If this potential drop is of suificient magnitude the ringer at the unsignaled station would operate to give a false signal. To insure against such false operation it is essential that the resistance of the line conductor be limited to such a value that 'the potential drop thereover, occurring under the condition described, is insufiicient to cause the ringer to operate.

It is the object of this invention to provide an improved signaling arrangement for two-party line circuits which materially increases the permissible length of line loop over which signaling may take place without encountering crossringing.

. This object is attained in accordance with a feature of the invention by simultaneously applying the usual source of signaling current to one of the line conductors, and a source of potential to the, other line conductor; the said'source of potential being of a magnitude just below that required to operate a ringer on zero loop, and degrees out of phase with the cross-ringing potential.

The invention will be readily understood from the following, detailed description made with reference to the accompanying drawing in which:

Fig. 1 isa circuit diagram. illustrating a twoparty line circuit arranged for selective signaling in accordancewith the features of the present invention; and

Fig. 2 is a circuit diagram illustrating a twoparty line circuit arranged for selective signaling in accordance with well-known practice.

The line L, Fig. 1, is shown extending from a central oflice and having connected thereto the substation equipments of stations A and B. The ringer at station A is connected between the ring conductor R and ground, while the ringer at station Bis connected between the tip conductor and ground. Atthe central ofiice there is provided a conventional two-party ringing key K having two operating positions. At S there is illustrated a source of signaling current having a lead extending therefrom to the upper contacts of key K. 'S represents an auxiliary source of signaling current comprising a transformer whose primary winding is connected between one terminal of the source S and ground and whose secondary winding is connected between the lower contacts of key K and ground.

When key K is actuated to the left, the source S of signaling current is connected to the tip conductor T of line L and the primary winding of the transformer of the source S" is connected to the ring conductor R of the line. Signaling current from source S traverses a path including the upper left contacts of key K, lead C, tip conductor 'I' of line L, to ground through the ringer at station B. The ringer at station B responds to this current in a well-known manner. The secondary winding of the transformer of source S" is adjustable and the adjustment is such that the potential connected between the ring conductor R and ground by way of the lower left contacts of key K and lead D is just insufficient to actuate a ringer on zero loop. Obviously,

therefore, the ringer at station A connected to 2 which shows a two-party selective signaling system of the type in common use today. This showing distinguishes from that of Fig. 1 only in that no auxiliary source of potential is provided at the central office. The operation of the system illustrated in Fig. 2 is identical to that of Fig. 1 except that when the source S is applied to either of the line conductors R or T by the operation of key K, ground alone is connected to the other conductor.

Assuming the station B has been signaled and the receiver thereat has been removed. from its switchho-ok, it will be observed that a path for the signaling current traversing the tip conductor T exists, which may be traced from the tip conductor T, through the now closed switchhook contacts at station B, ring conductor R of line L to ground through the ringer at station A; and that a parallel path to ground at the central ofiice extends from the point on conductor R at which theringer of station A is connected, over the ring conductor R, lead D and the lower left contacts of key K. The ringer at station A is therefore subjected to the potential drop over this latter path, and if the dropis of sufficient magnitude, the ringer will falsely operate. As the length of the line L is a factor which determines the potential drop thereover, it is apparent that this length must not exceed a value which will produce a drop of sufficient magnitude to operate the ringer at station A. The line loop over which signaling may take place without cross-ringing is thus limited.

Referring back to Fig. 1 and assuming, as in the case just described, that station E has been signaled and the subscriber thereat has removed the receiver from its switchhook while the key K is still operated, it will be observed that the potential drop (due to cross-ring current) over the lead R from the point thereon at which the ringer at station A is connected, to the central office, is opposed by the potential provided by the auxiliary source S", since this potential is, as shown, arranged to be 180 degrees out of phase with the cross-ringing potential. If we assume,

for example, that the ringer of stations A or B will just fail to operate on 40 volts applied directly to their terminals, that is, on zero loop,

it will be observed that the maximum permissible, line loop over which signaling may take place without introducing cross-ringing will be attained when the transformer of source S is adjusted to furnish just 40 Volts. If X is indicated as the length of line conductor in the old arrangement,

Fig. 2, over which signaling may take place without introducing cross-ring, that is, a length which will produce just less than 40 volts, the potential required to operate a ringer, then the length of line in Fig. 2 may be doubled before the crossringing condition manifests itself.

To further illustrate, if we assume a line loop of zero length in the system shown in Fig. 1, the potential of source 7 S" will be applied to the ringer at the unsignaled station, but since this potential is 40 volts, the potential at which a ringer will just fail to operate, the ringer will not respond. If we assume the line loop to be of X length, the potential applied to the ringer at the unsignaled station will be zero, that is, the 40 volt drop in the line conductor minus the 40 volts furnished by the source S". If we assume the line loop to be of 2X length, then the potential applied to the ringer at the unsignaled station will be 40 volts, that is, the drop of 80 voltsin the line conductor minus the 40 volts supplied by the source S".

From the foregoing description it is apparent that the present invention permits signaling over a much longer line than was heretofore possible without encountering the objectionable effects of cross-ringing.

What is claimed is:

1. In a signaling system, a telephone line having tip and ring conductors, a pair of subscriber stations on said line, each having a ringer connected between ground and a different one of said line conductors, and each requiring a potential of predetermined magnitude for its operation, a source of signaling current, a source of potential just below the predetermined magnitude required for the operation of said ringers, and means at a central office for simultaneously applying said source of signaling current to one of said line conductors and said source of potential to the other of said line conductors.

2. In a signaling system, a telephone line having tip and ring conductors, a pair of subscriber stations on said line, a ringer at one of said stations connected between ground and the ring conductor of said line, a ringer at the other of said stations connected between ground and the tip conductor of said line, each of said ringers requiring a potential of predetermined magnitude to effect its operation, a source of signaling current, a source of potential whose magnitude is less than the predetermined magnitude required to operate said ringers, and means at a central office for applying said source of signaling current to either the tip or ring conductors of said line and said source of potential to the other conductor of said line.

3. In a signaling system, a telephone line having a pair of stations thereon, a ringer at each station connected between ground and a different conductor of said line, and each ringer requiring -a potential of predetermined magnitude applied to its terminals to effect its operation, a source of signaling current, a source of potential whose magnitude is just less than the predetermined magnitude, and means for applying said source of signaling current to one conductor of said line and said source of potential to the other conductor of said line.

4. In a signaling system, a telephone line having a pair of stations thereon, a ringer at each station connected between ground and a different conductor of said line, and means for selectively operating said ringers comprising a grounded source of signaling current, a transformer primary winding connected between the ungrounded terminal of said signaling source and ground, a transformer secondary winding having one terminal grounded, and means for selectively connecting the ungrounded terminal ofsaid signaling source to, either of the conductors 'of said line and simultaneously connecting the ungrounded terminal of said transformer secondary winding to the otherconductor of said line.

5. In a signaling system; a' telephone line having a pair of stations thereon, av ringer at one station connected between ground and the tip conductor of said line, a ringer at the other station connected between ground and the ring conductor of said line, a source of signaling current at a. central oflice, meansv at the central oflice for connecting said source of signaling current to either of said lines to effect the selective operation of said ringersgmeans at one of said stations effective when operated in response to the'operae tion of the ringer thereat for establishing a return path to ground at the central oflice for the signaling current, in parallel with the ringer at the said one of said stations and for connecting the ringer at the other station in parallel with said return path, whereby the ringer at said other station is subjected to the potential drop over the said return path, and means for rendering the ringer at the said other station unresponsive to the potential drop over the return path, comprising a source of potential of predetermined magnitude at the central ofi'ice and means for connecting said source of potential to said return path in phase opposition to the potential drop therein.

6. In a signaling system for two-party telephone lines in which the signaling device at one station is connected between ground and the ring conductor of the line and the signaling device at the other staiton is connected between ground and the tip conductor of the line and in which means are provided for selectively connecting signaling current to either of said lines to effect the operation of the signaling device associated therewith and in which the removal of the receiver at the signaled station interconnects the tip and ring conductors of the line and effectively connects the signaling device of the unsignaled station to the line conductor over which signaling current is being transmitted whereby the signaling device of the unsignaled station is subjected to a cross-ringing potential, a source of potential and means for connecting said source of potential to the conductor other than the one to which the source of signaling current has been connected, in phase opposition to the cross-ringing potential, whereby the signaling device at the unsignaled station is'rendered unresponsive to the cross-ringing potential.

STANLEY F. NELSON. 

